Harvesting isn't a really clean operation, chaff and dirt aren't seperated well from most grains in favor of speed rather than cleaning. All that extra plant material is flammable.
i used to burn sugar by pouring it from those little packets in restaurants. But sugar dust is like lint, it just floats in the air forever.
The chaff grinds up when being transfered from place to place with the grain. Grain itself cracks and is pulverized into small dust particles. A very dry atmosphere and the fumes from insectides in these bins make it the explosive problem.
We used to actually add vegetable oil to grain to keep the dust from floating everywhere. If the stuff doesn't float in air it doesn't get lit.
Like anything else, operator failure is the prime cause for these explosions. The conveyors, screw augers, bin fans, etc. will get a bad bearing at some point. If they get hot enough they'll ignite the contents of their surroundings.